TT #377 · Parashas Shoftim · Seventh Aliyah · Passuk 21:06
And all the elders of that city who are nearest to the slain person shall wash their hands over the heifer whose neck was broken in the valley.
Torah Temimah points to Sifrei Devarim 209.
TT lemma: עַל הָעֶגְלָה הָעֲרוּפָה בַנָּחַל.
English: ‘Over the heifer whose neck was broken in the valley’ – at the place of the arifah.
Derash on the location of the washing (Sifrei) — it must be done over the animal in the valley.
The hand-washing takes place in the valley, over the body of the heifer that has just been killed.